The structure and assembly of the outer membrane of Escherichia coli is being studied. Lipopolysaccharide is inserted at points in the membrane and, after a period when it can diffuse, is organized into domains that cannot be penetrated with new molecules, as shown by density labeling, immunologic labeling, and electron microscopy. Electron spin resonance studies show that the polysaccharide confers rigidity upon the bilayer of this membrane. Mutants showing antibiotic hypersensitivity due to presumed outer membrane changes have been isolated, mapped, and characterized physiologically. Biochemical characterization is underway. Conjugation and transduction procedures have been worked out for a strain of E. coli of medical and biochemical interest which has heretofore been refractory to such analysis. The properties of a transducing phage carried by this strain have also been studied. BIBLIOGRAPHIC REFERENCES: Coleman, W.G., Jr., Goebel, P.J., and Leive, L. Genetic analysis of Escherichia coli 0111:B4, a strain of medical and biochemical interest. J. Bacteriol. 130: 656-660, 1977. Rottem, S., and Leive, L.: Effect of variations in lipopolysaccharide on the fluidity of the outer membrane of Escherichia coli. J. Biol. Chem. 252: 2077-2081, 1977.